Opiates are important for the treatment of pain, but their abuse presents a major health problem. Understanding opioid actions is crucial to their rational use clinically and the development of potential treatments for abuse. Opiates and the opioid peptides act through a family of receptors, of which a number have been cloned. Understanding how these receptors work at the molecular level and in vivo is crucial to enhance our ability to develop new agents to treat pain and to avoid the problems of abuse, as well as to better utilize the drugs currently available. This application is focused upon understanding the actions of these opioid receptors. Clinical experiences suggest that mu opiate analgesics differ pharmacologically, raising the possibility of multiple mu receptor subtypes. This possibility has also been suggested from detailed binding and pharmacological studies. More recently, we have identified a number of splice variants of the MOR-1 receptor in mouse, rat and humans. A major component of this application is based upon the characterization of these variants and understanding their relevance to opioid pharmacology. Receptor heterogeneity can also be achieved by dimerization. We will examine the interactions of MOR-1 and its splice variants with the mouse version of the orphanin FQ/nociceptin receptor (ORL1), termed KOR-3. Finally, there are a number of modulatory systems involved with opioid actions. These have the potential of enhancing the actions of opioids without a corresponding increase in side effects. Sigma1 receptors have been implicated in the modulation of opioid analgesia, but not gastrointestinal transit. Sigma1 receptor antagonists potentiate opioid analgesia and eliminate many of the differences in sensitivity towards opioids seen among strains of mice. Having now cloned the mouse and rat versions of the sigma1 receptor, we plan to explore its actions at the molecular and functional level. Finally, transporters such as P-glycoprotein play a major role in maintaining the blood-brain barrier. However, recent work suggests that they also are responsible for transporting neurotransmitters/modulators from the brain into the periphery, including a variety of opioids. We also will continue our investigations of these transporters on opioid action. By examining both the receptors themselves and modulatory systems, we hope to extend our understanding of opioid analgesics and gain insights into how to use them appropriately and minimize their abuse.